During normal human pregnancy maternal plasma osmolality decreases significantly a response which may be of value in protecting the mother and infant from maternal dehydration. Fetal plasma osmolarity also decreases in relation to newborn values. It is unknown whether the pregnancy factors which result in a decrease in maternal plasma osmolarity primarily reduce fetal osmolarity or whether the fetal plasma osmolarity decrease is purely secondary to equilibration in response to maternal hypotonicity. The purpose of this research is to define the normal pattern of water balance and osmoregulation in the postpartum mother and newborn infant by following measures of plasma composition.